A wide variety of solenoid actuators and solenoid actuated devices are known in the electromechanical arts. Solenoid actuators may be broadly classified as having dual-pole or single-pole solenoids. In most dual-pole solenoid designs, an armature is spaced at an axial air gap with a stator having a coil embedded therein. Dual-pole solenoids are often identified by an armature diameter that is about the same or greater than an outer diameter of the coil winding of the stator. When the coil in a dual-pole solenoid is energized, magnetic flux is generated around the coil, and flux lines pass through the stator, to the armature and back to the stator. The resulting flux path produces a pair of magnetic north and magnetic south poles between the stator and armature on each side of the axial air gap. The flux between these poles is generally parallel to the armature motion. These opposite poles produce a force on the armature that tends to move it toward the stator and coil to accomplish some task, such as opening or closing a valve.
In a typical single-pole solenoid, the magnetic flux path also encircles the coil and passes through the stator, the armature and back to the stator. The resulting flux path also produces a pair of magnetic north and south poles between the stator and the armature. In contrast to dual-pole configurations, the flux path between the poles is parallel to armature motion for one set of poles, and perpendicular to armature motion for the other set of poles. The perpendicular portion of the flux path may traverse a sliding radial air gap between the armature and another electromagnetic component that is present to complete the magnetic circuitry. Single-pole solenoids are often identified by an armature diameter that is smaller than the inner diameter of the coil winding of the stator. Due at least in part to manufacturing considerations, the additional electromagnetic component that is present in single-pole solenoids is often not a part of the stator itself. Rather, it is generally in contact with or positioned very close to the stator. The extra electromagnetic component is also typically stationary, hence the description of the air gap between this extra component and the armature as a “sliding radial air gap.” Single-pole solenoids remain preferred in certain applications.
The extra electromagnetic component mentioned above is often referred to as a magnetic flux ring. It is common to use materials such as iron and silicon iron to form the flux ring component. While such flux rings have worked well in certain designs, they are often associated with leakage of magnetic flux out of the desired flux path. In addition, because such flux rings are typically made of materials that are not only magnetically conductive, but also electrically conductive, eddy currents can form within the flux ring. The eddy currents create magnetic fields resisting the pull-in force on the associated armature when the solenoid is energized. Certain designs have been proposed to address these issues. Among them is the proposal to form slots in the flux ring to create longer path lengths for eddy currents to travel. As a result, the magnetic fields generated by the eddy currents may be relatively weaker. While these strategies have seen some success, there remains room for improvement.